To examine whether the genetic and environmental influences on substance consumption are ‘developmentally stable ’ or ‘developmentally dynamic’, we used the model presented in Fig. 1, which illustrates additive genetic sources only. This model is based on a common pathway model and has three main features. First, the model contains a substance consumption factor for each wave that encompasses those influences that impact on all three substances. There are paths from each of the common factors to the specific substances at each time wave that indicate the degree to which the liability for each specific substance is reflected by the common factor.
